Prostate cancer is the most common deadly cancer of men. Bone metastasis is a painful complication of advanced prostate cancer associated with significant morbidity. Endothelin-1 (ET-1) is a prostate cancer- secreted factor that activates the osteoblast endothelin A receptor (ETAR) causing osteoblast proliferation and pathologic new bone formation. In turn, osteoblasts send chemotactic and growth cues back to the prostate cancer cells. Preliminary data have unexpectedly demonstrated that ablation of both ET-1 and androgen action in osteoblasts is necessary to reduce bone lesion growth in castrate-resistant prostate cancer (CRPC). Furthermore, osteoblast generation of active androgens from adrenal dehydroepiandrosterone (DHEA) further fuels osteoblast-directed prostate cancer progression in bone. The goals of this proposal are to investigate the actions of ET-1 and androgen on tumor burden in an animal model prostate cancer bone metastasis in three specific aims. Aim 1 will examine the effects of castration and DHEA replacement combined with ETAR blockade in a ?humanized? animal model of prostate cancer bone metastasis. Scid mice will undergo castration or sham surgery, and treatment with the ETAR antagonist zibotentan or a vehicle control. The effects of sustained-release DHEA to negate the effects of combined zibotentan and castration on the development of prostate cancer skeletal lesions will be tested. It is expected that DHEA treatment, in castrated mice treated with ETAR blockade, will increase the development and/or size of prostate cancer lesions in bone compared to control mice not receiving DHEA. Aim 2 will examine the effects of Hsd3b7 knockout on the progression of prostate cancer bone lesions in DHEA-treated mice. It is hypothesized that the protein encoded by Hsd3b7 is responsible for osteoblast conversion of DHEA to androstenedione, and androstenediol to testosterone. It is expected that knockout of Hsd3b7 will negate the effects of DHEA on the development of prostate cancer lesions in castrated mice treated with ETAR blockade. Aim 3 will determine if combined ETAR blockade and androgen depletion prevents the initiation of skeletal lesions, the progression of established lesions, or both. The combination of castration and ETAR pharmacologic blockade reduced the number of skeletal lesions in a mouse model of bone metastasis compared to either alone. The timing of androgen depletion and ETAR blockade required to reduce skeletal tumor burden is unknown. Male scid mice will undergo castration or sham surgery before inoculation of tumor cells or at the point when skeletal lesions have been established. It is expected that the combination of androgen depletion and ETAR blockade will prevent both the initiation and the progression of established lesions. ADT is the standard treatment in men with metastatic prostate cancer, but disease progression to CRPC and bone metastases in most men mark the fatal form of the disease. The clinical implication of this research is that ETAR blockade may be effective only with maximal androgen blockade. These findings generated by this proposal will be a catalyst for clinical trials to reduce the morbidity and mortality of CRPC in veterans.